mkfs: terminate getsubopt arrays properly

mkfs: terminate getsubopt arrays properly

Having not drank any (or maybe too much) coffee this morning, I typed:

$ mkfs.xfs -d agcount=3 -d nrext64=0
Segmentation fault

I traced this down to getsubopt walking off the end of the dopts.subopts
array.  The manpage says you're supposed to terminate the suboptions
string array with a NULL entry, but the structure definition uses
MAX_SUBOPTS/D_MAX_OPTS directly, which means there is no terminator.

Explicitly terminate each suboption array with a NULL entry after
making room for it.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
[sandeen: explicitly add NULL terminators & clarify comment]
Reviewed-by: Eric Sandeen <sandeen@sandeen.net>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>